CEP-15: (C*) Add git submodule for Accord

As talked about in dev@ thread "Intra-project dependencies”, we talked about adding git submodules but before doing this had to work out a few issues first; this ticket is to track this work.

Goals

• when checking out an older commit, or pulling in newer commits, the submodule should also be updated automatically
• release artifact must include the submodule and must be able to build without issue
• build.xml must be updated to build the submodule
• build.xml must be updated to release the submodule jar